1. Field of the Invention
The present invention relates generally to mobile communication systems, and in particular, to an apparatus and method for transmitting and receiving signals using multiple antennas.
2. Description of the Related Art
In CDMA (Code Division Multiple Access) mobile communication systems, such as CDMA2000 systems or UMTS (Universal Mobile Telecommunications System) systems, multiple antennas are used to implement schemes for increasing transmission capacity on radio channels.
Schemes using multiple antennas are classified into multiplexing schemes and diversity schemes. In multiplexing schemes, parallel channels are set up between a transmitter and a receiver to transmit signals. In diversity schemes, a transmitter or a receiver uses multiple antennas to improve the performance of transmission or reception. The multiplexing scheme does not improve transmission/reception reliability since it cannot provide sufficient diversity effects, but increases a transmission rate. In addition to these schemes, an improved scheme is used in which multiple antennas are used along with channel coding techniques in order to improve transmission performance, i.e., decrease a transmission error rate, in a transmitter. As the number of antennas of the transmitter and the receiver in diversity schemes increases, however, the schemes does not increase transmission rate, but increases systems complexity.
In order to solve this problem, a scheme for dividing antennas of a transmitter into several groups for signal transmission has been proposed (see V. Tarokh, A. Naguib, N. Seshadri, A. R. Calderbank, “Combined array processing and space-time coding,” IEEE trans. On Information Theory, vol. 45, pp. 1121-1128, May 1999). The proposed scheme is disclosed in U.S. Pat. No. 6,127,971, entitled “Combined Array Processing and Space-Time Coding,” issued on Oct. 3, 2000. This scheme is advantageous in that the complexity of a receiver is not increased in a geometrical progression as the number of antennas increases, and a transmission rate of a signal can increase by separating a space channel into several subchannels.
However, in this scheme, when the number of transmission antennas is small, the number of antennas in each divided group also decreases. In addition, it is difficult to obtain multiplexing effects. Under such conditions, this scheme can obtain only a diversity effect. In addition, it is hard to obtain even the diversity effect, because when the number of transmission antennas is small, the number of antennas in each divided group is also small.